Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde.

We studied the threshold photoionization and dissociative ionization of para -, meta -, and ortho -anisaldehyde by photoelectron photoion coincidence spectroscopy in the 8.20-19.00 eV photon energy range. Vertical ionization energies by equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) calculations reproduce the photoelectron spectral features in all three isomers. The dissociative photoionization (DPI) pathways of para - and meta -anisaldehyde are similar and differ markedly from those of ortho -anisaldehyde. In the para and meta isomers, the lowest-energy DPI channel corresponds to hydrogen atom loss to form the C 8 H 7 O 2 + fragment at m / z 135, which undergoes sequential dissociation processes at higher energies, such as carbon monoxide loss to C 7 H 7 O + ( m / z 107) and further, sequential CH 3 , CH 2 O, and CH 2 CO losses to produce C 6 H 4 O + ( m / z 92), C 6 H 5 + ( m / z 77), and C 5 H 5 + ( m / z 65), respectively. Carbon monoxide loss from the parent ions, yielding C 7 H 8 O + ( m / z 108), is a subordinate dissociation channel parallel to H atom loss. At higher energies, it also gives rise to sequential formaldehyde (CH 2 O) loss to produce C 6 H 6 + ( m / z 78). In the ortho -anisaldehyde cation, the vicinity of the aldehyde and methoxy groups opens up low-energy hydrogen-transfer processes, which allow for seven fragmentation channels to compete effectively with the H- and CO-loss channels. Thus, the fragmentation mechanism changes considerably, thanks to the steric interaction of the substituents. Functional group interactions, in particular H transfer pathways, must therefore be considered when predicting the isomer-specific unimolecular decomposition mechanism of cationic and neutral species, as well as mass spectra for isomers.